A printed circuit board (PCB) includes a layout of the circuit pattern designed on an insulating substrate in accordance to the connecting requirement of the electronic components fixed thereon. Therefore, the insulating substrate must undergo a certain machining process and surface treating process to form the circuit pattern. The circuit pattern can be formed on one side surface or double side surfaces of the layers in order to provide electrical interconnection among the electronic components. One type of printed circuit board is generally called “flexible printed circuit board”, which includes at least one flexible substrate and which is light in weight, thin and is bendable, thereby providing several specific features.
The flexible printed circuit board is used widely in the notebook computer, LCD device, digital cameras, cellular phones and several consumer electric instruments. The manufacture of the electronic device is in the trend to produce in compact size, the intended region for layout of circuit traces in the flexible printed circuit board becomes lesser due to the increased number circuit traces in order to complement with the increased electronic components. In addition, flexibility of the printed circuit board is a must in order to dispose the same within a limited space of the compact size.
FIG. 1A shows a sectional view of a conventional flexible printed circuit board 10, and includes a substrate 11, upper and lower adhesive layers 12 disposed on top and bottom surfaces of the substrate 11 for holding upper and lower conductive layers 13, 13a. And upper and lower cap layers 14, 14a disposed above to cover the upper and lower conductive layers 13, 13a respectively via additional adhesive layers 12. The upper and lower conductive layers 13, 13a are generally made from thin copper foil. Since the upper and lower cap layers 14, 14a cover the entire surfaces of the upper and lower conductive layers (13, 13a), the conventional printed circuit board 10 has a relative stiffness and consequently provides little flexibility. As shown in FIG. 1B, the conventional double sided printed circuit board generally defines a foldable region 15, wherein a portion (in registry with the foldable region) of the upper conductive layer is removed in order to enhance the flexibility thereof, so that the conventional printed circuit board 10 can be folded when required into a configuration, as best shown in FIG. 1C.
With the increasing of circuit traces and electronic components, and the reducing of the layout area, the circuit traces must be formed on both sides of the substrate in the foldable region 15. The result is that the conventional printed circuit board has a considerable large stiffness.